Methodology for a Safety Case of a Dual Purpose Cask for Storage and Transport of Spent Fuel | IAEA

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International Atomic Energy Agency Vienna

Methodology for a Safety Case of a Dual Purpose Cask for Storage and Transport of Spent FuelIAEA-TECDOC-1938

Methodology for a Safety Case of a Dual Purpose Cask for

Storage and Transport of Spent Fuel

Report of a WASSC/TRANSSC Joint Working Group

IAEA-TECDOC-1938

IAEA-TECDOC-1938

IAEA TECDOC SERIES

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IAEA SAFETY STANDARDS AND RELATED PUBLICATIONS

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METHODOLOGY FOR A SAFETY CASE OF A DUAL PURPOSE CASK

FOR STORAGE AND TRANSPORT

OF SPENT FUEL

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AFGHANISTAN ALBANIA ALGERIA ANGOLA

ANTIGUA AND BARBUDA ARGENTINA

ARMENIA AUSTRALIA AUSTRIA AZERBAIJAN BAHAMAS BAHRAIN BANGLADESH BARBADOS BELARUS BELGIUM BELIZE BENIN

BOLIVIA, PLURINATIONAL STATE OF

BOSNIA AND HERZEGOVINA BOTSWANA

BRAZIL

BRUNEI DARUSSALAM BULGARIA

BURKINA FASO BURUNDI CAMBODIA CAMEROON CANADA

CENTRAL AFRICAN REPUBLIC CHADCHILE CHINA COLOMBIA COMOROS CONGO COSTA RICA CÔTE D’IVOIRE CROATIA CUBACYPRUS

CZECH REPUBLIC DEMOCRATIC REPUBLIC

OF THE CONGO DENMARK DJIBOUTI DOMINICA

DOMINICAN REPUBLIC ECUADOR

EGYPT EL SALVADOR ERITREA ESTONIA ESWATINI ETHIOPIA FIJIFINLAND FRANCE GABON

GEORGIA GERMANY GHANA GREECE GRENADA GUATEMALA GUYANA HAITI HOLY SEE HONDURAS HUNGARY ICELAND INDIA INDONESIA

IRAN, ISLAMIC REPUBLIC OF IRAQIRELAND

ISRAEL ITALY JAMAICA JAPAN JORDAN KAZAKHSTAN KENYA

KOREA, REPUBLIC OF KUWAIT

KYRGYZSTAN

LAO PEOPLE’S DEMOCRATIC REPUBLIC

LATVIA LEBANON LESOTHO LIBERIA LIBYA

LIECHTENSTEIN LITHUANIA LUXEMBOURG MADAGASCAR MALAWI MALAYSIA MALIMALTA

MARSHALL ISLANDS MAURITANIA

MAURITIUS MEXICO MONACO MONGOLIA MONTENEGRO MOROCCO MOZAMBIQUE MYANMAR NAMIBIA NEPAL

NETHERLANDS NEW ZEALAND NICARAGUA NIGER NIGERIA

NORTH MACEDONIA NORWAY

OMANPAKISTAN PALAU PANAMA

PAPUA NEW GUINEA PARAGUAY

PERUPHILIPPINES POLAND PORTUGAL QATAR

REPUBLIC OF MOLDOVA ROMANIA

RUSSIAN FEDERATION RWANDA

SAINT LUCIA

SAINT VINCENT AND THE GRENADINES SAN MARINO SAUDI ARABIA SENEGAL SERBIA SEYCHELLES SIERRA LEONE SINGAPORE SLOVAKIA SLOVENIA SOUTH AFRICA SPAIN

SRI LANKA SUDAN SWEDEN SWITZERLAND

SYRIAN ARAB REPUBLIC TAJIKISTAN

THAILAND

TOGOTRINIDAD AND TOBAGO TUNISIA

TURKEY

TURKMENISTAN UGANDA UKRAINE

UNITED ARAB EMIRATES UNITED KINGDOM OF

GREAT BRITAIN AND NORTHERN IRELAND UNITED REPUBLIC

OF TANZANIA

UNITED STATES OF AMERICA URUGUAY

UZBEKISTAN VANUATU

VENEZUELA, BOLIVARIAN REPUBLIC OF

VIET NAM YEMEN ZAMBIA ZIMBABWE The following States are Members of the International Atomic Energy Agency:

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IAEA-TECDOC-1938

METHODOLOGY FOR A SAFETY CASE OF A DUAL PURPOSE CASK

FOR STORAGE AND TRANSPORT OF SPENT FUEL

REPORT OF A WASSC/TRANSSC JOINT WORKING GROUP

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For further information on this publication, please contact:

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December 2020

IAEA Library Cataloguing in Publication Data Names: International Atomic Energy Agency.

Title: Methodology for a safety case of a dual purpose cask for storage and transport of spent fuel / International Atomic Energy Agency.

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FOREWORD

Spent nuclear fuel generated in the operation of nuclear reactors needs to be safely managed following its removal from the reactor core. Reactor storage pools were designed based on the assumption that, after a short period of time, spent nuclear fuel would be removed for reprocessing, disposal or storage elsewhere. Owing to delays in making decisions on the disposition of spent fuel and in putting decisions into effect, the volume of highly radioactive spent fuel that needs to be stored is growing and additional storage capacity is required.

One widely used option for additional storage capacity is the use of dry spent fuel storage casks.

Of the various existing dry storage concepts, several Member States are using the dual purpose cask. There are obvious benefits to storing spent fuel in a container that can be safely handled and stored, and that provides levels of radiation shielding, heat dissipation, criticality safety and containment making it acceptable for transport in the public domain. However, these benefits come with inherent strategic risks that need to be managed over the entire storage timescale.

In April 2011, the IAEA initiated a working group to develop guidance for Member States on an integrated safety case for dual purpose casks for the transport and storage of spent fuel, with the support of both the Transport Safety Standards Committee (TRANSSC) and the Waste Safety Standards Committee (WASSC). This publication is based on discussions within the TRANSSC/WASSC Working Group during its activities from 2011 to 2013. It provides information on the structure and contents of an integrated safety case for a dual purpose cask.

The publication is expected to be of interest to designers, vendors, operators, licensees, regulators, technical support organization and others involved in the development and review of the safety case and supporting safety assessment.

The IAEA appreciates the contributions of various experts to this publication. The IAEA officers responsible for this publication were Y. Kumano, A. Guskov and S. Whittingham of the Division of Radiation, Transport and Waste Safety.

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EDITORIAL NOTE

This publication has been prepared from the original material as submitted by the contributors and has not been edited by the editorial staff of the IAEA. The views expressed remain the responsibility of the contributors and do not necessarily represent the views of the IAEA or its Member States.

Neither the IAEA nor its Member States assume any responsibility for consequences which may arise from the use of this publication.

This publication does not address questions of responsibility, legal or otherwise, for acts or omissions on the part of any person.

The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries.

The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA.

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CONTENTS

INTRODUCTION ... 1

1. BACKGROUND ... 1

2. WORKING GROUP ACTIVITIES ... 2

3. OBJECTIVE AND SCOPE ... 2

4. DEFINITIONS ... 4

5. STRUCTURE OF THIS PUBLICATION ... 4

PART 1: GENERAL PRINCIPLES AND TECHNICAL INFORMATION ... 6

1.1. TRACKING THE HISTORY OF DPCSC ... 6

1.2. BASIC ADMINISTRATIVE AND TECHNICAL INFORMATION ... 6

1.3. SPECIFICATION OF CONTENTS ... 7

1.4. SPECIFICATION OF THE DPC ... 8

1.5. STORAGE AND TRANSPORT CONDITIONS ... 9

1.5.1. Basic concept ... 9

1.5.2. Operational scenarios ... 11

1.5.3. Operational scenarios impact ... 18

1.6. GENERAL DESIGN CONSIDERATIONS AND ACCEPTANCE CRITERIA ... 25

1.6.1. Relationship between regulatory requirements, performance criteria, acceptance criteria, design criteria, and design specification ... 25

1.6.2. Basic design prerequisites ... 28

1.6.3. Performance criteria ... 29

1.6.4. Design principles and acceptance criteria ... 30

1.7. AGEING CONSIDERATIONS ... 39

1.7.1. Introduction ... 39

1.7.2. Components and ageing mechanisms to be considered ... 39

1.7.3. Component evaluation ... 43

1.7.4. Preshipment inspection after storage period ... 49

1.8. COMPLIANCE WITH REGULATORY REQUIREMENTS ... 52

1.8.1. Transport package design approval and storage licensing period 52 1.8.2. License types for storage ... 53

1.9. OPERATION ... 53

1.10. EMERGENCY PLAN ... 54

1.11. MANAGEMENT SYSTEMS ... 54

1.11.1.Maintenance plan ... 54

1.11.2.Lessons learned from literature on ageing management ... 55

1.11.3.Essence of the systematic approach to ageing management ... 56

1.11.4.Ageing management programme for DPC storage facilities ... 58

1.11.5.DPCSC periodic review ... 60

1.11.6.Record management ... 60

1.12. DECOMMISSIONING ... 61

PART 2: SPECIFIC TECHNICAL ASSESSMENT ... 62

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2.1.3. Analysis of DPC design ... 63

2.1.4. Comparison of acceptance criteria with results of analysis ... 63

2.2. STRUCTURAL ANALYSES ... 64

2.2.1. Reference to DPC design for structural analysis ... 64

2.2.2. Assumptions for structural analysis ... 64

2.2.3. Description and validation of methods for structural analysis .... 64

2.2.4. Structural assessment ... 66

2.3. THERMAL ANALYSES ... 68

2.3.1. Reference to DPC design for thermal analysis ... 68

2.3.2. Assumptions for thermal analysis ... 68

2.3.3. Description and validation of methods for thermal analysis ... 68

2.3.4. Thermal assessment ... 70

2.4. ACTIVITY RELEASE ANALYSIS ... 71

2.4.1. Reference to DPC design for activity release analysis ... 71

2.4.2. Assumptions for activity release analysis ... 71

2.4.3. Description and validation of calculation method for activity release analysis ... 72

2.4.4. Activity release calculations ... 73

2.5. EXTERNAL DOSE RATES ANALYSIS ... 79

2.5.1. Reference to DPC design for dose rates analysis ... 79

2.5.2. Assumptions for dose rates analysis ... 79

2.5.3. Description and validation of calculation method for dose rates analysis ... 80

2.5.4. Dose rate calculations ... 80

2.6. CRITICALITY SAFETY ANALYSIS ... 82

2.6.1. Reference to DPC design for criticality safety analysis ... 82

2.6.2. Assumptions for criticality safety analysis ... 82

2.6.3. Description and validation of the calculation method for criticality safety analysis ... 83

2.6.4. Criticality safety calculations ... 84

REFERENCES ... 87

DEFINITIONS ... 93

ABBREVIATIONS ... 97

ANNEX ... 99

CONTRIBUTORS TO DRAFTING AND REVIEW ... 107

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INTRODUCTION

This introduction provides the background and history of the Working Group activities as well as general discussion to consider for subsequent parts of this report (PART 1 and PART 2).

1. BACKGROUND

Operating nuclear reactors generate spent fuel, which needs to be safely managed following its removal from reactor cores. The reactor on site pool-type storage capacities were designed based on the assumption that fuel would be removed after a certain period of time either for reprocessing, disposal, or further storage. However, as a result of storing higher burn-up fuel, significantly increased timeframe till disposal solutions are prepared, and delays in decisions on strategies for spent fuel management, the volume of spent fuel discharged from reactors which needs to be managed and stored is on the increase. Consequently, additional storage capacity may be needed following the initial storage in reactor pools.

In some countries, a concept of dual purpose cask (DPC) is considered as an option for further storage. This is because of that the concept increases flexibility for storage capacity, as well as its economic efficiency that can reduce the complexity of handling highly radioactive spent fuels.

The primary safety objectives of a DPC design relate to national storage regulations and compliance with the transport regulations extant at the time of transport. DPCs are generally designed with a dual containment boundary and are designed and maintained so the primary lid need not be opened for inspection or maintenance during storage or before transport after storage to avoid unnecessary degradation, incidental risks, and radiological exposures. Storage based on this concept basically does not require additional equipment (such as hot cells).

If a DPC is designed based on a single-containment boundary concept, it is necessary to provide appropriate maintenance facilities that can be used to maintain the cask in the event of failure of primary containment boundary.

Managing spent fuel using a DPC involves storage and on-site and off-site transport of the spent fuel before and after storage. Many countries require licenses for storage of the spent fuel in the DPC or for storage facilities containing DPC packages. Most countries also require package design approval for the DPC package to be transported.

Safety assessment and approval or licensing procedures have to consider the differences between the two DPC configurations (i.e. the DPC transport package design and the DPC storage package design). A DPC provided for transport is usually equipped with impact limiters and often has a one-lid closure system. The acceptance criteria for this DPC transport package are defined in Ref [1]. The DPC transport package also needs to be designed so that it can be used in an operational mode that is different from usual transport packages. More specifically, the DPC transport package needs to be transported after several decades of storage and, therefore, needs to use long term resistant packaging components that require ageing considerations.

A DPC package provided for storage is usually not equipped with transport impact limiters, but

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activities, including storage and on-site transport, and they are very often different from SSR-6 (Rev. 1) requirements. Nevertheless, most of the safety relevant DPC components are the same for both purposes.

Therefore, demonstration of compliance of the DPC package with national and international transport regulations, as well as with the storage requirements in an integrated manner is recommended.

2. WORKING GROUP ACTIVITIES

The International Conference on Management of Spent Fuel from Nuclear Power Reactors, which was hosted by the IAEA in June 2010, recommended establishing a joint international working group to provide guidance to Member States for integrating the safety cases for storage and transport of spent fuel in a DPC in a holistic manner. A consultancy meeting (CS-130) was convened to “Establish a Working Group on an Integrated Safety Demonstration for the Dual Use Cask for Spent Nuclear Fuel” at the IAEA in November 2010. The meeting also developed the terms of reference for that working group.

The objectives of the working group were:

(1) To prepare an IAEA guidance document (TECDOC or Safety Report) containing guidance for the structure and contents of a DPC integrated safety case (DPCSC) (as a supporting document to Refs [2-5]);

(2) To provide recommendations to the Transport Safety Standards Committee (TRANSSC), Waste Safety Standards Committee (WASSC), Radiation Safety Standards Committee (RASSC), and Nuclear Safety Standards Committee (NUSSC), as appropriate, for changes to be made to existing IAEA requirements and guidance relevant to the licensing and use of transport and storage casks for spent fuel.

Plenary meeting for the working group meetings were held at the IAEA Headquarters in April 2011 (TM-40975), April 2012 (TM-42920), and April 2013 (TM-44985).

The working group took Ref. [6] as an initial model regarding structuring of the guidance. The work was distributed into 4 sub-groups.

3. OBJECTIVE AND SCOPE

This TECDOC contains guidelines for the structure and contents of a DPCSC. The scope is only for dual-purpose metal storage and transport casks for short- and long-term dry storage (as defined in Ref. [4], Appendix I). This publication does not cover requirements for a safety case of a DPC storage facility. A canister is considered a DPC component when it is contained within a DPC as a part of its internals.

This TECDOC aims to assist designers, vendors, operators, licensees, regulators, technical support organizations, and others in developing and reviewing the safety case and supporting safety

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Reference [2] (see also Ref. [7]) introduces the concept of safety case as follows:

The safety case is a collection of arguments and evidence in support of the safety of a facility or activity. The safety case will normally include the findings of a safety assessment, and will typically include information (including supporting evidence and reasoning) on the robustness and reliability of the safety assessment and the assumptions made therein.

An integrated safety case for transport and storage aims to support the application for the package design approval for transport and the application for the licensing of the storage cask (as part of the safety case for the storage facility). The DPCSC may be a collection of scientific and technical arguments including safety assessments in support of:

(1) The demonstration of compliance with Ref. [1] for off-site transport, including transport after storage;

(2) The demonstration of compliance with the international standards and national requirements for dry storage of spent fuel as they apply to the DPC package during its storage period.

This TECDOC is based on the concept of an integrated DPCSC. This concept assumes that the DPCSC is in line with Ref. [4] and linked to the transport and storage approvals as described in subsequent paragraphs (see also Figure 1).

The basic information for the DPCSC is the description of the DPC and its contents, the impact conditions and acceptance criteria. The term ‘impact conditions’ means all basic data for the safety assessment arising from normal, off-normal, and accident conditions of storage and routine, normal, and accident conditions of transport (RCT, NCT, and ACT). Transport regulations provide impact conditions for off-site transport. The impact conditions for storage need to be specified based on national regulations and an assessment of the operational conditions at the storage facility.

‘Acceptance criteria’ are based on regulatory limits that the DPC package and the storage facility are required to meet (e.g. dose rates). The acceptance criteria for off-site transport are given in the transport regulations. The acceptance criteria for storage (to be applied to each DPC package/storage facility combination) need to be specified based on national regulations and an assessment of the operational conditions of the storage facility. This basic information is complemented by instructions for operation and maintenance. The DPCSC needs then to demonstrate that a DPC of the specified design loaded with the specified contents and being exposed to the defined impact conditions, operations, and maintenance meets the specified acceptance criteria. A regulatory body could assess this demonstration leading to an approval of the DPC package design. Assuming approval will be given only if compliance with the transport regulations has been demonstrated in the DPCSC, the design can be approved as a transport package. Regarding storage, the DPCSC could qualify the DPC package for storage in a specific facility.

This concept leaves some freedom to the DPC designer in defining impact conditions and acceptance criteria. In either case, the transport requirements are not so flexible and need to be met.

An incorrect choice of storage impact conditions or acceptance criteria could lead to problems in

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conditions and acceptance criteria have to be selected based on a careful review of the regulatory requirements and operational limits and conditions of the storage facility. Of course, acceptance criteria can also be set in a more restrictive manner, which should provide some additional margin in assessing current and future storage facilities.

4. DEFINITIONS

The definitions included in Refs [1, 7]apply throughout this publication. The definitions section toward the end of this publication provides additional publication-specific definitions.

5. STRUCTURE OF THIS PUBLICATION

Part 1 provides a generic consideration of the organization and contents of a DPCSC. It also provides information on administrative matters; specification of contents; DPC specifications, DPC performance criteria; and compliance with regulatory requirements, operation, maintenance, and management systems as a part of the DPCSC.

Part 2 provides generic and specific considerations for technical assessments of the safety case.

Figure 1 shows the structure of the DPCSC.

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PART 1: GENERAL PRINCIPLES AND TECHNICAL INFORMATION Part 1 of the DPCSC needs to include the following information.

1.1. TRACKING THE HISTORY OF DPCSC

As soon as a DPC has its own life cycle starting from design and ending with decommissioning, a DPCSC is a ‘rolling process’ and is updated periodically, or when incorporating new findings.

Therefore, it is important to clearly identify exact stage of the life cycle and the issue version of each DPCSC document or subdocument and keep updated a list of DPCSC documents, including a description of each document version.

1.2. BASIC ADMINISTRATIVE AND TECHNICAL INFORMATION

The DPCSC include the following basic administrative and technical information:

(1) Designer-specific model identification of the DPC.

(2) Identification of DPC designer (name, address, contact details).

(3) Type of transport package.

(4) Transport-specific limitations of operational conditions after short- or long-term storage, e.g.:

(a) Modes of transport for which approval is requested;

(b) Any special instructions to the carrier such as required special transport configurations (e.g. transport frame, canopy).

(5) Storage specific limitations of operational conditions for generic DPC package licenses, e.g.:

(a) Need for storage building;

(b) Environmental conditions (temperature, wind, snow, etc.);

(c) Storage orientation (vertical, horizontal);

(d) Handling capacity (weight, dimensional limits);

(e) Fuel retrievability (hot cell, etc.);

(f) Maintenance/repair capability;

(g) Inspection and maintenance frequency;

(h) Storage pitch (minimum distance between DPC packages);

(i) Accident conditions (drop height/orientation, tip over, tornado, missile, flooding. etc.);

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(j) Siting requirements, including seismic, tsunami, and volcano;

(k) Monitoring requirements.

(6) Reference to applicable transport regulations and/or storage requirements, including the edition of IAEA Regulations for the Safe Transport of Radioactive Material and other relevant IAEA Safety Standards to which the DPC design refers.

(7) List of laws, regulations, guidelines, codes, standards, and licenses applicable to the design, fabrication, quality assurance programme, transport, and storage of the DPC package based on the defined operational scenarios, as well as the related nuclear facilities and modes of transport to be used. From these laws, regulations, and guidelines, the regulatory requirements (technical, operational, and other) that control the design, analysis, and operation of the DPC package need to be determined and included in the DPCSC. These regulatory requirements have to be tabulated and presented in Section 1.8 with a description of the design, safety analysis results, and references to DPCSC sections.

(8) Reproducible conceptual drawings need to be provided. The conceptual drawings may include bird’s eye views and three dimensional illustrations showing the configuration of the DPC in each transport and storage modes indicating the major components of the DPC, such as packaging, impact limiters, devices for thermal insulation, and packaging inserts, if applicable. The illustrations need to indicate at least the overall outside dimensions, the masses of the main components of the packaging, and the gross mass for empty and loaded conditions.

1.3. SPECIFICATION OF CONTENTS

A detailed description of the permitted radioactive contents of the DPC needs to include, but is not limited to, the following information, as applicable:

(1) Radionuclides / radionuclide composition; progeny, if applicable.

(2) Activity, mass and concentrations, and heterogeneities, if applicable.

(3) Physical and chemical state, geometric shape, arrangement, loading restrictions, irradiation parameters, moisture content, and material specifications (particularly, information on spent fuel degradation during storage).

(4) Fuel condition (e.g. damaged, non-damaged, intact, or consolidated fuel rods; fuel assemblies with missing rods,). Fuel integrity may be defined in the national regulations or guidelines (e.g. Ref. [8]) or based on international technical reports (e.g. Ref. [9]).

(5) Nature and characteristics of the radiation emitter. (6) Thresholds of heat generation rate for contents.

(7) Mass of fissile material or fissile nuclides.

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(8) Other contents such as canisters and non-fuel hardware (e.g. control rods, sources, thimble plugs, burnable poison rods, moisture absorbers, etc.).

(9) Typical parameters of spent fuel which provide the basis for the derivation of some of above descriptions, such as fuel design type, initial enrichment, burnup and cooling period.

(10) The acceptable parameters of the history of the spent fuel before loading. Before it is loaded in the DPC, the fuel will have been subjected to a number of processes, including irradiation in the reactor, handling operations and pool storage, all of which can influence the physical integrity of the fuel rods and the structural components. The history of the spent fuel before loading is, therefore, an important input into the safety case.

1.4. SPECIFICATION OF THE DPC

The DPC design has to be defined by including the following information, as applicable:

(1) A list of all DPC components, monitoring systems, and complete design drawings for transport and storage configurations;

(2) A parts list of all safety related components including bolts and seals;

(3) Material specifications of all DPC components and standard items and methods of their manufacture including requirements for material procurement, welding, other special processes, non-destructive evaluation, and testing;

(4) Information on material degradation during storage and transport;

(5) A description of:

(a) The DPC body, lid (closure mechanism) and inserts;

(b) The DPC components of the containment system;

(c) The DPC components required for shielding;

(d) The DPC components for criticality control;

(e) The DPC components for thermal protection;

(f) The DPC components for heat dissipation;

(g) The protection against corrosion;

(h) The protection against contamination;

(i) The transport configuration, including any devices required for the transport including impact-limiting components, canopies and tie-downs, which may have an effect on the

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(j) The storage configuration, including any devices required for the safe handling and storage that may have an effect on the safety of the package in storage operations.

1.5. STORAGE AND TRANSPORT CONDITIONS

This section needs to describe the performance criteria that allow the DPC design to meet applicable transport regulations and the storage safety requirements such as summarized here:

(1) Radioactive material containment;

(2) Shielding (control of external radiation levels);

(3) Criticality prevention;

(4) Heat removal (prevention of damage caused by heat);

(5) Stored spent fuel retrievability;

(6) Structural integrity;

(7) Ageing.

For this purpose, the DPC designer has to first consider DPC package operational scenarios, and has to identify the regulatory and licensing requirements. The designer has to then develop operational procedures for each operational step included in the scenarios, and identify conditions to which the DPC package could be subjected considering the operational limits. Furthermore, the designer needs to describe analysis assumptions and data used for the safety case and how they are derived from the design and the behaviour of the package under routine, normal, and accident conditions of transport (RCT, NCT, and ACT) and normal, off-normal and accident conditions of storage. This is especially true regarding the release of radioactive material, radiation levels, criticality safety, heat removal, structural integrity of the DPC, and integrity of contained spent fuel.

This section needs to include items to be considered in developing the DPCSC, from the determining the operational scenario to interpreting the safety analysis basis.

1.5.1. Basic concept

When developing the DPCSC, the DPC designer first determines the DPC package operational scenarios by considering:

(1) Operational scenarios

The DPC designer has to consider DPC package operational scenarios, including those in the DPCSC, together with the nuclear facilities (either actual or postulated) related to each scenario.

The DPC designer also needs to justify each operational scenario (e.g. country specific requirements, regulatory situation, siting, technical feasibility, safety philosophy) selection in the

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(2) Safety case for DPC package storage system

A complete safety case for the DPC package storage system will be achieved by integrating the DPCSC and related nuclear facilities safety cases. Thus, the DPC designer and nuclear facility operator responsibilities need to be agreed upon before developing the safety case. Therefore:

(a) In general, safety cases related to DPC package operations in a given nuclear facility have to be included in the nuclear facility safety case, as the safety analysis or assessment and the associated acceptance criteria depend on the environmental conditions unique to that facility.

(b) In some cases, normal operations (e.g. loading, unloading and handling of DPC packages) and off-normal operations (e.g. operations during loss of power, loss of crane operation) in nuclear facilities are specific to the DPC design. In such cases, the safety cases related to the operations involving the DPC package at the storage facility may also be included in the DPCSC.

(c) Nuclear facilities accidents, except those incidents that are considered and for which acceptance criteria are defined, are to be considered by the nuclear facilities.

(3) Environmental conditions

Some Member States provide a regulatory framework of regulations or guidelines that stipulates environmental conditions to be considered at the storage facility for the DPC package storage design. This allows approval of the DPC package design independently of the storage site. Storage facility operators may select a DPC design that fits their site conditions from approved designs or design a storage facility to meet selected DPC design specifications. In the latter case, the DPCSC can include the safety assessment of the DPC package in the specified storage environment.

(4) Time spans

The DPC designer has to consider the intended storage and transport time span.

(5) Operational procedures and environmental conditions of operation.

The DPC designer has to develop procedures for each step in the considered operational scenarios and include them in the DPCSC. At the same time, environmental conditions of the DPC package operations have to be clearly defined and included in the DPCSC. The developed operational procedures have to be presented in Section 1.9, “Operation” in the DPCSC.

(6) Retrievability

Retrievability of the DPC content is required under Ref. [2], requirement 11, and specifically addressed in Ref. [4], paras 6.133 and 6.134.

In this publication, retrievability is the ability to recover DPC contents. Some states may define the condition at retrieval.

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(7) Retrieval Facility

The storage safety may not rely as heavily on the previous operational steps if a retrieval facility has the necessary infrastructure to enable opening a DPC for inspection of the internals and the spent fuel in the DPC. The same is true if the storage facility allows for the opening of the lid for DPC maintenance and repair work.

Inspection of spent fuel and DPC internals demonstrates the storage safety at the storage facility and ensures the safety of transport after storage and safety of spent fuel retrieval at the next destination facility.

1.5.2. Operational scenarios

1.5.2.1. Operational steps that constitute the operational scenario

The DPC operational scenario consists of various steps addressed in the DPCSC. The DPC designer has to select and organize them sequentially from the following list of steps.

(1) DPC package preparation (for transport and storage, including spent fuel loading and inspections);

(2) On-site transport (before storage and/or after storage);

(3) Off-site transport (before storage and/or after storage);

(4) Handling at storage facility (before and after storage);

(5) Storage (on-site or off-site);

(6) DPC package unloading (at the destination of transport after storage).

Figure 2 illustrates DPC operational steps, including some of their required elements; Figures 2a-2d shows typical operational scenarios.

The DPCSC will be clear about which of the possible various operational scenarios need to be included and, in addition to transport, the approval being sought. The operator is responsible to ensure operations that are carried out, but not within the scope of the DPCSC, are adequately covered elsewhere (e.g. within the storage and/or retrieval facility safety case).

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12

FIG. 2. Transport/ storage operational steps.

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FIG. 2a. Scenario for on-site storage operational steps. FIG. 2b. Scenario for off-site storage operational steps.

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c. Scenario for on-site and off-site storage operational steps. FIG. 2d. Scenarios for on-site and off-site repair routines.

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1.5.2.2. Notes on each operational step

Section 1.5.2.1 states that the DPC designer has to develop operational procedures and include the environmental conditions of operations in the DPCSC. Some guidance for developing the operational procedures is addressed as follows:

(1) DPC package preparation:

(a) This step is in principle conducted at the spent fuel storage pool at nuclear power stations.

(b) To initiate this step, the DPC has to be fabricated as designed and the spent fuel to be loaded complies with the DPC spent fuel specifications. It needs to be ensured that the operator of this step confirms the former by the record of fabrication inspections supplied by the DPC vendor and the latter by the record of nuclear plant fuel inspections.

 Under the operational scenario where there is no inspection of DPC internals by removal of the DPC lid(s) (such as after storage in preparation for shipment), the condition of the spent fuel and the DPC package preparation confirmed in this step provide initial conditions for the safety assessment in all of the following operational steps. The spent fuel and the DPC, therefore, need to be properly inspected, recorded, and referenced in the following steps.

 This step includes preparing the DPC for spent fuel loading, lid(s) closure, internal water drainage, drying, inert gas filling, preparation for transport, and preshipment inspections. Detailed preparation and inspection procedures may differ for on-site or off-site transport of the DPC package.

(2) On-site transport:

(a) On-site transport is necessary at all facilities involved in the scenario.

(b) On-site transport may consist of any movement of the DPC package at nuclear facilities where the off-site transport regulations usually do not apply. Such on-site transport may include transfer between different nuclear facilities/buildings as long as public roads or railways transport are not involved.

(c) On-site transport begins when the DPC package is ready for on-site transport in the nuclear facility dispatching the DPC package, and ends when the DPC package is unloaded in nuclear facility receiving the DPC package.

(d) Generally, environmental conditions and the configurations of the DPC package between on-site and off-site transport will differ.

 Compared with off-site transport, on-site transport environmental conditions tend

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controlled operations. It may not be the case, however, that off-site transport environmental conditions bound those of on-site transport.

 While during off-site transport a DPC package is generally secured horizontally in or on a conveyance with impact limiters attached, on-site transport may be conducted without impact limiters, or vertically.

(3) Off-site transport:

Off-site transport of the DPC package is conducted in compliance with Ref. [1] or similar national regulations. Environmental conditions of off-site transport are prescribed in the transport regulations, and the safety assessment of the DPC package under those conditions has to be included in the DPCSC. The DPC package condition prior to transport after storage relies on safe storage at the facility.

(4) Storage facility handling:

(a) There are generally two steps to handling of the DPC package at a storage facility: i) handling in preparation for storage and ii) handling in preparation for transport after storage. For installations equipped for fuel retrieval, additional handling steps to prepare transfer of the DPC package between the storage position and the retrieval installation needs to be considered.

(b) While preparing for storage, a receipt inspection needs to confirm whether the DPC package complies with storage limits and conditions of the facility. Then operations of configuration changes from transport to storage (i.e. removal of impact limiters), and DPC package transfer to and storage at the storage location are conducted.

(c) Though preparation for shipment is the reverse of preparation for storage, a preshipment inspection to confirm whether the DPC package complies with the transport regulations after the storage period, instead of the receipt inspection that is completed prior to storage, will be conducted.

(d) Consideration needs to be given to all situations in which handling mechanisms could malfunction.

(e) Consideration has to be given to the possibility of DPC package becoming wedged and immovable within the spent fuel storage facility. In addition to the issue of shielding in such circumstances, consideration needs to be given to whether handling equipment and systems are able to recover from such situations or could be damaged by the application of excessive stresses.

(5) Storage:

(a) The safety of storage relies on the proper preparation of the DPC package for storage, its safe transport to the storage facility, and maintaining specified environmental

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(b) There are generally two options for storage: i) on-site storage and ii) off-site storage.

For an on-site storage facility located inside the boundary of a nuclear power station site, the DPC package would be shipped to the next destination (e.g. a spent fuel handling facility for unloading) by off-site transport after storage at the facility. For the off-site storage option, a DPC package is first transported from a nuclear power plant to an off-site storage facility, and may be transported again to a subsequent destination (perhaps for reprocessing or disposal) after storage.

(c) A design option for some storage facilities is to construct a storage building, which mitigates impacts from natural phenomena to the DPC package and reduces the radiation level at the site boundary by the shielding provided by the building structure.

Incidents such as building collapse or a cooling air inlet blockage need to be considered.

(d) Providing a fuel retrieval capability is another option for a storage facility design. When fuel retrieval capability is available, spent fuel can be unloaded from a damaged or otherwise compromised DPC to repair it, or fuel could be moved to another DPC. This capability allows for contingencies in the case of incidents and/or accidents.

Furthermore, to confirm post-storage shipment requirements compliance, spent fuel and DPC internals can be inspected by opening the DPC package. This reduces the reliance on fuel records management from previous steps, including storage.

(e) A hot cell is typical of a fuel retrieval installation. For on-site storage facilities, it may be possible to use the spent fuel storage pool at a nuclear power plant on-site as a retrieval installation. However, in the case of long-term storage for a period such as 50 to 100 years, the guaranteed period of availability of the pool has to be identified in the operational scenario. Alternative measures need to be provided if this guaranteed period is not possible. Alternative measures to control undue leakage of the first lid include DPC design features such as providing for a second lid qualified for off-site transport, or attaching a third lid (welded or bolted) to re-establish a double-barrier storage closure system, or to transport the DPC to another facility with a pool or a hot cell.

(f) When no spent fuel retrieval capability is available at the storage facility, there is no chance to directly confirm the state of the DPC internals or the spent fuel contained in the DPC after loading until the DPC package is unloaded at the destination facility. As confirming the DPC maintains its safety functions and verifying the status of the spent fuel at each operational step is essential, alternative inspection or assessment confirmation methods need to be established and described.

(6) DPC package unloading:

(a) DPC package unloading will be conducted at a reprocessing facility, nuclear power plant, another spent fuel storage facility, or the disposal facility. As this DPCSC concerns dual purpose casks (i.e. transport and storage), and not multi-purpose casks (i.e. transport, storage, and disposal), the DPC package has to be unloaded at the

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(b) Spent fuel retrieval safety at subsequent facilities relies on safe storage in the original storage facility and safe transport to the destination facility.

(c) The operational steps for DPC package unloading are the reverse of the DPC loading.

Two optional methods to unload spent fuel from DPC include wet unloading in a pool and dry unloading within a hot cell. The latter eliminates processes such as water injection into DPC package, spent fuel reflooding, and placement of DPC package into water.

1.5.3. Operational scenarios impact

1.5.3.1. Incidents considered for each operational scenario

To establish conditions with which to design the DPC and to assess its safety, the DPC designer needs to postulate conditions that the DPC package may encounter at each operational step in the operational scenarios defined in Sections 1.5.1 and 1.5.2, and identify every loading (mechanical, thermal, radiological, chemical, electrical, etc.) that could have an adverse effect on the DPC and its contents as impact conditions. The DPCSC needs to identify and justify reasons for selecting operational situations and related impact conditions .

Safety arguments concerning outside the regulatory environment of transport or storage facility or a storage site design basis accident are out of the scope of the DPCSC. However, when it is a matter of public or competent authority’s concern, such arguments may be included.

(1) DPC package preparation

Designed DPC package preparation operations including handling inside the loading facility (nuclear power plant) are considered normal conditions. Incidents caused by a credible single failure of equipment or a credible single human error are considered to be off-normal conditions.

Accidents in the facility, such as a DPC package drop inside/outside the reactor building, are out of the scope of the DPCSC (but in the scope of the facility safety case).

(2) On-site transport

Transport regulations cover situations to be considered during on-site transport. When on-site transport is conducted under conditions not covered by the off-site transport regulations, or if the DPC configuration is different than for off-site transport (e.g. without impact limiters or transport in vertical orientation of the DPC), normal, off-normal, and accident conditions of on-site transport have to be defined commensurate to frequencies of occurence and consequences of the credible incidents/accidets.

(3) Off-site transport

Reference [1] prescribe three conditions for classifying off-site transport situations: i) RCT (incident free), ii) NCT (minor mishaps), and iii) ACT (credible accidents).

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(4) Storage facility handling

Planned DPC package handling operations for storage preparation, shipment preparation and inspection, or DPC maintenance if applicable during storage are considered normal conditions.

Incidents caused by minor mishaps, a credible single equipment failure or a credible single human error are considered to be off-normal conditions. Incidents such as a tip over or drop of the DPC package, or a fall of an overhead crane onto the DPC package can be classified as accident conditions.

(5) Storage

The facility operator needs to identify situations or incidents during storage to be evaluated, as they are specific to the facility siting and design and to DPC package operation in the facility.

Reference [4], Annexes V and VI provide comprehensive examples of anticipated incidents in spent fuel storage facilities. For some Member States, national spent fuel storage regulations or guidelines, such as Refs [10–12], define incidents and accidents to be considered in the design of the storage facility.

As a DPC is a static component stationary during storage with its safety functions maintained statically, nothing would happen under normal conditions of storage, except a self-induced phenomenon (i.e. ageing). DPC package environmental conditions, including effects of natural events, will differ depending on storage location (indoors or outdoors).

Incidents caused by minor mishaps, a credible single failure of equipment, or a credible single human error are considered to be off-normal storage conditions. Situations caused by postulated initiating events, such as credible equipment failure, operator or human induced error, or natural events have to be identified and classified with careful consideration to their occurrence frequencies and consequences as either off-normal or accident conditions during storage. In some Member States, an aircraft crash and consequent building collapse and fire has to be considered as an example of human induced accident. In other States, less frequent but extreme natural events such as tsunami or volcanic eruption may have to be considered. Even a hypothetical radioactive material release from the loss of containment of a single DPC package due to non-mechanistic reasons can be considered accident conditions to demonstrate safety of storage.

(6) DPC package unloading

Planned DPC package unloading operations, including handling inside the unloading facility, are considered normal conditions. Incidents caused by minor mishaps, a credible single failure of equipment, or a credible single human error are considered off-normal conditions.

1.5.3.2. Loading factors impacting the DPC

Any loading impacting the DPC in each operational step including the most severe natural loadings at the storage facility need to be considered with reference to historical records and siting investigations of the storage facility site and its surrounding area. Seismic loading needs to be established according to the approach discussed earlier.

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Examples of conditions to be considered are:

(1) Mechanical loadings:

(a) Internal and external pressure;

(b) Dead load, compressive load by stacking;

(c) Bolt tightening load, reaction load from seals;

(d) Thermal stress by expansion or contraction;

(e) Transport acceleration, vibration, handling acceleration (lifting, rotating);

(f) Impact load due to drop or collision; local load at collision area;

(g) Impact load by a heavy item dropped onto the DPC; or by collision of a wind driven missile, a turbine missile, or an aircraft crash; local load at the point of impact;

(h) Seismic load, tsunami load, wind load, snow load.

(2) Thermal loadings:

(a) Ambient temperature, solar insolation;

(b) Deformation or dimensional change caused by thermal expansion or contraction;

(c) Thermal load by fire;

(d) Thermal load from peripheral DPC packages;

(e) Temperature rise by vacuum drying or blockage of cooling air inlet;

(f) Thermal shock by reflooding of DPC internal;

(g) Material structure change, decomposition by heat, and thermolysis gas;

(h) Ageing, including creep, stress relaxation, and overageing.

(3) Radiological impacts:

(a) Hardening or embrittlement of metal or polymers by radiation;

(b) Material structure change, decomposition by radiation, radiolysis gas;

(c) Loss in efficiency of built-in neutron absorbers.

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(4) Electrochemical or chemical reactions:

(a) Electrochemical or chemical reactions between different materials, reaction products;

(b) Corrosion, stress corrosion cracking (SCC), corrosion products.

1.5.3.3. Example of impact conditions

Table 1 presents examples of situations and conditions used for designing and assessing a DPC package derived from typical operational scenarios. This example is based on a DPC design under the following conditions:

 The DPC package is stored inside a storage building or on a storage pad outdoors.

 No spent fuel retrieval installation is available in the storage facility. Therefore, the storage facility is designed to prevent the DPC and its contents from damage inhibiting the ability of the safety functions to comply with the transport regulations during storage and handling at the facility.

 According to national regulations, off-site transport approval includes on-site transport conducted in conjunction with off-site transport.

Table 1, rows 1 and 2 show typical examples of incidents and accidents that are to be considered, but not limited to, for off-normal conditions and accident conditions. Credible incidents and accidents in the DPCSC have to be carefully selected considering national storage regulations.

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TABLE 1. SITUATIONS AND LOADING TO BE CONSIDERED IN EACH OPERATIONAL STEP

No. Classifications Conditions Loading

(a) Preparation and loading

1 Normal conditions

(i) Pressurization for drainage;

(ii) Internal vacuum;

(iii) Internal temperature rise;

(iv) Transfer inside the facility.

Internal/external pressure Dead load

Bolt tightening load Seal reaction load Lifting load Transferring load Thermal load Ambient temperature

2 Off-normal

conditions (to be considered in the facility’s safety case) ―

3 Accident conditions (to be considered in the facility’s safety case) ―

(b) Off-site transport

4 RCT

(i) Transport:

- Ambient temperature of -40°C to 38°C;

- Solar insolation;

- Handling and transport acceleration.

(ii) External pressure of 25 kPa.

Bolt tightening load Seal reaction load Lifting load Transporting load Vibration Impact load Thermal load Ambient temperature

5 NCT

(i) Water spray;

(ii) 0.3 m drop;

(iii) Stacking;

(iv) Steel bar drop;

(v) Ambient temperature of -40°C to 38°C.

Bolt tightening load Seal reaction load Stacking load Local load Impact load Thermal load Ambient temperature Insolation

Irradiation (a.s.) Ageing (a.s.)

6 ACT

(i) 9 m drop;

(ii) 1 m drop onto steel bar;

(iii) Fire (800°C, 30 minutes);

(iv) 15 m immersion;

(v) 200 m immersion.

Bolt tightening load Seal reaction load Local load Impact load Thermal load Insolation

Heat input form fire Irradiation (a.s.) Ageing (a.s.)

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TABLE 1. (Continued)

(c) Handling at storage facility

7 Normal operation

(i) Lifting acceleration:

- Ambient temperature and pressure;

- Lifting acceleration.

(ii) Transfer inside the facility:

- Ambient temperature and pressure;

- Transferring acceleration.

Bolt tightening load Seal reaction load Lifting load Transferring load Thermal load Ambient temperature Irradiation (a.s.) Ageing (a.s.)

8 Off-normal

conditions

Minor collision with peripheral equipment (e.g. transport frame) or surrounding DPC packages

Bolt tightening load Seal reaction load Impact load Thermal load Ambient temperature Irradiation (a.s.) Ageing (a.s.)

9 Accident conditions (i) Tip over.

(ii) Drop from handling height.

Bolt tightening load Seal reaction load Impact load Thermal load Ambient temperature Irradiation (a.s.) Ageing (a.s.) (d) Storage

10 Normal conditions

(i) Storage;

Ambient temperature and pressure Solar insolation, wind, rain, snow

(outdoor storage) (ii) Ageing.

Bolt tightening load Seal reaction load Securing load Thermal load Ambient temperature Irradiation

Ageing

11 Off-normal

conditions

(i) Natural events:

- Earthquake, flood;

- Tornade (outdoor storage);

- Blockage of cooling air (in-building storage);

(ii) Human induced events:

- Power source failure.

Bolt tightening load Seal reaction load Seismic load Thermal load Ambient temperature Irradiation

Ageing

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TABLE 1. (Continued)

(d) Storage (continued)

12 Accident conditions

(i) Extreme natural events:

- Earthquake, tsunami, flood, volcanic eruption;

- Wind driven missiles.

(ii) Human induced events:

- Tip over;

- Gas explosion;

- Aircraft crash;

- Fire.

(iii) Release of radioactive material (form single DPC with non-mechanistic reason).

Bolt tightening load Seal reaction load Thermal load Ambient temperature Irradiation

Ageing

(e) Unloading

13 Normal conditions (i) Pressurization during filling water;

(ii) Internal vapour and water;

(iii) Internal temperature decrease;

(iv) Transfer inside the facility.

Bolt tightening load Seal reaction load Lifting load Transferring load Thermal load Ambient temperature Irradiation

Ageing

14 Off-normal

conditions Blockage of exhaust

Bolt tightening load Lifting load Thermal load Ambient temperature Irradiation

Ageing

15 Accident conditions (To be considered in the facility’s safety case) ―

* a.s.: after storage.

** Ageing includes creep, stress relaxation and overageing.

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1.6. GENERAL DESIGN CONSIDERATIONS AND ACCEPTANCE CRITERIA

When applying the concept of DPC system, safety assessment and approval or licensing procedures have to consider the differences between the two DPC configurations (i.e. the DPC transport package design and the DPC storage package design). The elements of the storage regime, the storage environment, monitoring/inspection, records, that are required to demonstrate compliance with the transport safety case needs be clearly stated in the safety case in compliance with the transport regulations, such that those designing the storage facility and those operating it can clearly understand what has to be implemented in the storage regime and provide the necessary records for future transport that this criterion has been achieved.

In this section, how regulatory requirements for both transport and storage are incorporated with DPC design is described.

1.6.1. Relationship between regulatory requirements, performance criteria, acceptance criteria, design criteria, and design specification

Regulations require the designer to meet ‘performance criteria’ for DPC transport packages and DPC packages used solely for storage (e.g. sufficient shielding, activity release limitations, criticality prevention, and sufficient heat removal). These performance criteria are connected to acceptance criteria. Acceptance criteria are derived from quantitative regulatory limits of performance criteria such as international and national regulations, standards, and requirements The engineering process for DPC design and technical assessment is the foundation for transport and storage design specifications.

The DPC design has to meet appropriate ‘design criteria’ (e.g. maximum allowable stress for a specified material under a specified loading condition) under the applicable operational or accident conditions as part of the design assessment for each DPC component and the assembled DPC.

The design, justified by technical assessment, is defined in a ‘design specification.’

Figure 3 shows how the design specification has to encompass the acceptance criteria for transport and storage. The transport package design acceptance criteria are derived from the international and national transport regulations, whereas the acceptance criteria in relation to the storage regulation is derived from international standards and national regulations. In addition, acceptance criteria for the DPC need to consider requirements that are specific to storage facility design. More detailed consideration on determining acceptance criteria is given in section 1.6.4. Figure 4 shows the relationship between various elements of the design process.

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FIG. 3. Relationship between design specification and acceptance criteria.

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